How does one denote the number of elements of a set $X$?

Question

If I have a specific set of some elements like:

$$X = \{1,2,3,4,5,6\}$$

How can one make an expression for the number of elements of the set $X$?

Answer 1

You're looking for the cardinality of a set, which is usually denoted by vertical bars:

$$| X |$$

Answer 2

It is commonly denoted with vertical or double vertical lines: $|X|$ or $||X||$. Once I also saw an accent: $\bar{X}$.

Answer 3

It's common enough to write $\#X$, too.

Answer 4

I have aso seen $n(X)$ (in Herstein I believe).